Discontinuously or periodically operating centrifuges are much used for producing sugar. We are concerned here with the processing step in which a sugar massecuite is spun off in a rotating centrifuge drum. In connection therewith, the centrifuge drum has a cover screen through which the syrup separated from the massecuite passes whereafter it then enters a centrifuge housing, in which the centrifuge drum is arranged, from the openings in the casing of the centrifuge drum.
The crystals released from the syrup in this way are then washed in the centrifuge drum with water or a highly purified syrup from a subsequent method step and finally removed from the centrifuge drum at the end of the separation process by a scraping device.
Thus, in the course of the process, the consistency and the composition of the liquid which passes through the cover screen changes. Firstly, there is a so-called green run-off which contains a high proportion of non-sugar material, i.e. it has a comparatively low sugar content.
Subsequently, so-called white run-off emerges through the cover screen and this has a substantially greater sugar content than the green run-off from the first process step. The white run-off occurs when the crystal layer on the cover screen is first sprayed with water thereby rinsing out the residual syrup, and sugar crystals are dissolved and forced through the permeable casing of the centrifuge drum due to the centrifugal force.
Finally, after these steps, a third liquid that is nevertheless similar to the white run-off passes through the casing, namely, when the residues still adhering to the centrifuge drum are rinsed off with washing water after the process of peeling off the sugar.
All three components of the discharge mentioned above are valuable and can be further processed However, the composition thereof is so different that greatly differing processes are more appropriate for the subsequent treatment. Thus for example, the white run-off and the sugar substance referred to as the third liquid that is dissolved by the washing water can frequently be returned to the centrifuge drum at the same stage, perhaps during the next or next-but-one discontinuously effected processing step namely, in place of the washing water.
This is not possible, or at least is not appropriate for the green run-off. This is expediently fed back into the cycle for the production of sugar massecuites during one of the preceding stages or is processed in a different manner due to the high proportion of non-sugar material.
It would therefore be desirable if these discharges could be separated from one another.
This desire has indeed been in existence for a long time. Thus, DE-patent 95 969 has already proposed the provision in a centrifuge housing of a separator which has a plurality of drainage channels at differing heights with separate discharge openings in each case. The discharge openings are then closed independently of each other and the discharges of differing composition are thereby separated and removed.
In order to improve upon this method, DE-patent 109 702 proposes that a valve be utilised and that the actuation thereof should effect the separation process.
In addition, P. W. van der Poel, H. Schiweck and T. Schwartz in “Zuckertechnologie. Rüben and Rohrzuckerherstellung”, Berlin (2000) at page 868 have proposed various measures for separating the green run-off and the white run-off immediately following it from each other by means of flaps or pivotal devices.
All these measures are confronted by the problem that the consistency of the white run-off and the green run-off is different and both do not impinge and then run off the inner wall of the centrifuge housing centrally in one position but do so over a 360° circular periphery, and they inevitably mix on their way from the centrifuge housing to the discharge point. The actual separation that is aimed for and desired does not occur and can at best lead to a fraction having a higher proportion of white run-off and a fraction having a lower proportion of white run-off.
A significant qualitative improvement becomes possible by using a proposal from DE 197 31 097 C1. Here an annular shutoff member having an external operating mechanism is arranged in the centrifuge housing near to the base. By appropriate actuation from the exterior, the time point at which the transition from the green run-off to the white run-off occurs can be matched exactly so that from this moment onwards the further drainage path of the syrup is changed by means of a lever mechanism in the interior of the centrifuge housing, i.e. the green run-off and the white run-off are diverted successively into different channels. The mixing process is thereby reduced and the separation process is improved.
Alternative proposals using shutoff members or channeling systems in the interior of the centrifuge housing are also known from DE 197 23 601 C1 and DE 100 02 862 A1
These proposals do indeed improve the quality but nevertheless they are mechanically complex and very difficult to construct and therefore expensive. Moreover, they also require regular maintenance, especially cleaning which is correspondingly difficult due to the arrangement thereof in the interior of the centrifuge housing and in addition they require the system to be stopped and therefore involve a time consuming temporary stoppage of the entire centrifuge so that the useful operational period thereof is limited accordingly.
It would be desirable, if instead, a process of separating the different kinds of syrup with acceptable quality but with lower constructional complexity were possible.
Consequently, the object of the present invention is to propose a device with the aid of which acceptable quality of the separation process is possible but with a lesser degree of constructional complexity.